Electronic equipment such as computer apparatuses and communication apparatuses is generally configured to be installed with a printed wiring board, on which components such as through-hole mounted devices are mounted. Through-hole mounted devices are connected to desired conductive layers in the printed wiring board in such a way that a lead wire for soldering is inserted into a through hole provided in the printed wiring board and soldered as is.
On the other hand, as the functionality of electronic equipment becomes increasingly sophisticated in recent years, multi-layered configurations of the printed wiring board to be installed in electronic equipment have been increasingly used.
In general, as shown in FIG. 1, multiplayer printed wiring board 101 has a multilayer structure comprising multiple layers including signal layer 102, power supply layer 103, and ground layer 104 as inner layers and surface layer 105. Herein, there is shown as an example, a multilayer printed wiring board of a 10-layer structure comprising four signal layers 102, one power supply layer 103, three ground layers 104, and two surface layers 105.
For example, through holes 106a and 106b are formed so as to penetrate the multiple layers and respectively connect to power supply layer 103 and ground layer 104. Through holes 106a and 106b are inserted with lead wirings 108a and 108b of through-hole mounted device 107. Through holes 106a and 106b are used to connect the terminals of through-hole mounted device 107 with power supply layer 103 and ground layer 104 as corresponding conductive layers, and to mount through-hole mounted device 107 by soldering.
That is, lead wiring 108a is used as the contact for power supply connection and lead wiring 108b is used as the contact for ground connection. It is noted that numeral 109 in FIG. 1 indicates solder.
When performing soldering, after lead wirings 108a and 108b of through-hole mounted device 107 are inserted into through holes 106a and 106b, batch soldering (dip soldering) by use of a flow apparatus or soldering by use of a soldering iron are performed.
The finished condition of soldering affects the connection reliability of the connection point of through-hole mounted device 107 and thereby affects the reliability of the entire electronic equipment in which multilayer printed wiring board 101 including through-hole mounted device 107 is installed.
One causal factor that can degrade the finished condition of soldering is a decline in the ability of molten solder to rise, in which the solder does not rise up to the ends of through holes 106a, 106b because of insufficient temperature rise during soldering. When a high quality soldering cannot be achieved throughout within through holes 106a, 106b because of the above described reason, the connection strength becomes insufficient thereby adversely affecting connection reliability.
Therefore, in order to achieve a satisfactory soldering within through holes 106a and 106b, it is necessary to sufficiently raise the temperature of the connection point by soldering with respect to the melting temperature of the solder.
For example, when heating is performed at the lower ends of through holes 106a, 106b, the heat is transferred to each conductive layer (signal layer 102, power supply layer 103, and ground layer 104) via the interior of through holes 106a, 106b. 
In particular, a solid pattern made up of copper foil is formed throughout the entire surfaces of power supply layer 103 and ground layer 104, and therefore heat is diffused from through holes 106a, 106b which are connected to power supply layer 103 and ground layer 104. Since power supply layer 103 and ground layer 104 have a large heat capacity, the temperature of through holes 106a and 106b is not likely to rise during soldering. Thereby, molten solder does not rise up to the ends of through holes 106a, 106b thereby causing contact failures.
Further, as described above, the number of components mounted onto multilayer printed wiring board 101 has increased recently, the number of wirings for connecting components has also been increasing. Along with this trend, the number of layers and the thickness of multilayer printed wiring board 101 has also been increasing, and the increase in the number of layers causes an increase in heat capacity of multilayer printed wiring board 101.
In articular, the number of layers of the multilayer printed wiring board in a large scale computer apparatus etc. may be several tens of layers. As the number of layers increases, heat conduction paths are increased thereby making it difficult to raise the temperature in the vicinity of through holes 106a and 106b. 
For this reason, as shown in FIGS. 2 and 3, thermal lands 111a and 111b for suppressing heat dissipation are formed around through holes 106a and 106b of the solid pattern of the plurality of inner layers including power supply layer 103 and ground layer 104 (for example, see Japanese Patent Laid-Open No. 09-008443, and Japanese Patent Laid-Open No. 2005-012088).
Thermal land 111b formed in ground layer 104 includes innermost peripheral region 112b and peripheral edge region 114b as shown in FIG. 2. Innermost peripheral region 112b is formed into a substantially circular ring shape around through hole 106b. Peripheral edge region 114b is formed in the peripheral edge part of innermost peripheral region 112b. Peripheral edge region 114b has spoke 113b for limiting heat conduction path, in the direction away from through hole 106b (along the radial direction with the center point of through hole 106b acting as the reference point).
In peripheral edge region 114b, there are formed spokes 113b as heat conduction paths at an equal angular spacing (at a spacing of 90 degrees) radially in four directions from innermost peripheral region 112b, and thus non-copper-foil regions 115b are formed in which copper foil is etched out in an arc shape leaving spokes 113b. 
This arrangement will result in heat being conducted to the solid pattern region surrounding thermal land 111b via spokes 113b from innermost peripheral region 112b with the heat conduction path being limited in four directions as shown by the arrow in the figure. This configuration will decrease the heat capacity of through hole 106b so that the temperature inside through hole 106b can be raised to a sufficient temperature for soldering.
Further, there are formed signal wirings 102e, 102f and signal wirings 102g, 102h, 102i each of which has a predetermined width (and whose paths are shown by a dotted line in FIG. 2), in signal layers 102 which are formed immediately above and immediately below ground layer 104.
In this respect, signal wirings 102e and 102f and signal wirings 102g, 102h and 102i are respectively formed in signal layer 102. Moreover, signal wirings 102e and 102h are wired bypassing non-copper-foil region 115b. Signal wirings 102f and 102g are wired passing through non-copper-foil region 115b. 
Thermal land 111a formed in power supply layer 103 includes a substantially circular ring shape innermost peripheral region 112a around through hole 106a, and peripheral edge region 114a as shown in FIG. 3. Peripheral edge region 114a includes spokes 113a for limiting the heat conduction path in the direction away from through hole 106a (along the radial direction with the center of through hole 106a being the reference point) in the peripheral edge part of innermost peripheral region 112a. 
In peripheral edge region 114a, there are formed spokes 113a as heat conduction paths at an equal angular spacing (at a spacing of 90 degrees) radially in four directions from innermost peripheral region 112a, and thus non-copper-foil regions 115a are formed in which copper foil is etched out in an arc shape leaving spoke 113a. 
This arrangement will result in heat conducted to the solid pattern region surrounding thermal land 111a via spokes 113a from innermost peripheral region 112a with the heat conduction path being limited in four directions as shown by the arrow in the figure. This configuration will decrease the heat capacity of through hole 106a so that the temperature within through hole 106a can be raised to a sufficient temperature for soldering. That is, through hole 106a will heat up more readily.
Further, there are formed signal wirings 102a, 102b and signal wirings 102c, 102d each of which having a predetermined width (and whose paths are shown by a dotted line in FIG. 3) in signal layers 102 which are formed immediately above and immediately below power supply layer 103.
A pair of signal wirings 102a and 102b and a pair of signal wirings 102c and 102e are respectively formed in signal layer 102. Moreover, signal wiring 102b is wired bypassing non-copper-foil region 115a. Signal wiring 102c is wired passing through non-copper-foil region 115a. It is noted that the pair of signal wirings 102a and 102b and the pair of signal wirings 102c and 102d form differential signal transmission lines.
The first problem to be solved, according to the above described related arts is that, the temperature rise during soldering is still not sufficient and the finished condition of the soldering is degraded thereby reducing connection reliability.
That is, as the functionality of electronic equipment becomes increasingly sophisticated, the related thermal land technology cannot cope with the further increase in the number of layers of the multilayer printed wiring board, and the heat conduction path will be increased making it difficult to raise the temperature in the vicinity of a through hole.
Moreover, further increase in the thickness of the multilayer printed wiring board results in longer through holes making the soldering to the interior of the through hole more difficult.
Further, when a lead-free solder (for example, tin-silver-copper lead-free solder) is used due to a compulsory requirement, a problem arises in that the melting temperature increases by several tens of degrees. That is, while the melting point of the relevant eutectic solder is 183 degrees C., the melting point of a lead-free solder, though this depends on its composition, is generally around 210 to 220 degrees C.
Therefore, when a lead-free solder is used for soldering, since heating the soldering point up to 240 to 260 degrees C. is required, the ability of molten solder to rise tends to further decline making it difficult to achieve sound soldering.
Further, the second problem, according to the above described related arts is that, since current will flow in the spoke of the thermal land, such a narrow width portion will provide a high resistance thereby causing the current capacity to decline.
That is, in the related thermal land, in order to decrease the heat capacity of a through hole, it is necessary to form non-copper-foil region (the region which is not the solid region) to be larger to some extent. However, when the printed board is used after package components are mounted thereon via lead wires, current flows through a spoke which has a locally narrow width and a large resistance, causing heat to be generated. This will lead to a decrease of current capacity and may cause the printed board to be burnt out.
Further, a third problem is that according to the above described related arts, signal transmission performance is degraded and the signal wiring density declines in signal wirings which are formed in the vicinity of through holes in the signal layers which are opposite to the power supply and ground layers formed with solid patterns via the insulation layer.
For example, as shown in FIG. 2, in the related thermal land 111b, non-copper-foil region 115b is formed in a circular ring shape around through hole 106b. Signal wirings 102f, 102g formed in a straight line has a portion which passes through non-copper-foil region 115b in ground layer 104 as an opposing solid layer. Because, in this portion, signal wiring 102f, 102g will have lost opposing ground layer 104, this will end up to cause disturbance in signal transmission.
In particular, for example as shown in FIG. 3, when a differential signal transmission line is formed by a pair of signal wirings 102c and 102d, for example, only one signal wiring 102c will pass through non-copper-foil region 115a. This would cause a difference in electric properties between the pair wirings thus causing degradation of signal transmission performance.
Further, for example as shown in FIG. 2, when disposing signal wiring 102h to avoid non-copper-foil region 115b so as to be opposite the copper foil region (solid region), the clearance between itself and adjacent signal wiring 102i will decrease thus causing the generation of cross-talk noise. Further, attempts to ensure enough clearance tend to result in a decrease in the wiring density of signal wiring.
Further, for example as shown in FIG. 3, in the case of the pair of signal wirings 102a and 102b forming a differential signal transmission line, disposing signal wiring 102b to avoid non-copper-foil region 115a so as to oppose the copper foil region (solid region) will result in a difference in the wiring length between signal wirings 102a and 102b thus causing generation of noise.
Further, a fourth problem is that, according to the above described related arts, the evenness of the conductive layer in the vicinity of a through hole degrades.
That is, in the related thermal land, a non-copper-foil region of a relatively large area is disposed in a circular ring shape around a through hole. In this non-copper-foil region, a signal layer, power supply layer, or a ground layer as a conductive layer (metal layer) located immediately below and above the aforementioned region via an insulation layer will be deflected. This tends to cause degradation of the evenness of the conductive layer of a printed wiring board and variations of the property of the printed wiring board.